Illuminating Objects Archive

Illuminating Objects intern Natasha Gertler tells us about her research.

The art of identifying decorative stones, such as those in the Courtauld’s Baroque frame, is achieved through observation, connoisseurship and comparison with other stones of known identity, rather than scientific analysis. Composed of exactly 1000 polished stone slabs of uniform shape and size, the extensive Corsi collection was established in Rome in the first quarter of the 19th century and is now housed in the Oxford University Museum of Natural History. Most of the stones are of known provenance and hence the collection is an excellent resource for decorative stone identification.

As soon as we were introduced to the collection, we immediately contacted Monica Price, the author of the comprehensive Corsi collection website as well as the Head of Earth Collections at the Oxford University Museum of Natural History. She kindly agreed to visit the Courtauld to view the frame and her extensive knowledge of decorative stones proved to be invaluable.

What was previously thought to be a wine-coloured variety of jasper, Monica identified to be in fact most likely an inlay of the mineral amethyst backed with a red metallic foil. This became very apparent when the segment was illuminated and viewed through a microscope, producing a reddish orange glow through the translucent purple amethyst, as shown below.

Amethyst probably backed with red metallic foil

Amethyst probably backed with red metallic foil, illuminated and viewed through a microscope

From a very early stage of my Illuminating Objects internship, I decided to display the frame alongside specimens of rocks and minerals that correspond to the stones present in the frame. I wanted to do so in order to emphasise the beautiful results of the interplay of man and nature, by highlighting the stones’ transformations from rough to cut and polished, revealing the true beauty of nature’s wonders.

Having already accumulated samples of lapis lazuli and amethyst from private collections in London, Monica kindly offered to lend us a variety of Sicilian jaspers and agates from the Earth Collection of the Oxford University Museum of Natural History. Sicilian jaspers are now depleted and no longer quarried, so it is very fortunate to be able to include a selection in my display.

Enthusiastically accepting her generous invitation, I readily booked my train ticket and off to Oxford I went. The stone slabs of the Corsi collection are splendidly preserved in drawers away from public display, and remain in Corsi’s original arrangement. His arrangement is very significant as he was one of the few collectors of his time who attempted to order their collection according to the geological properties of the stones rather than superficially by aesthetic similarities.

The Corsi collection is magnificent. Almost bursting in awe, I perused the stones, locating brilliant samples of my favourite rocks and minerals whilst discovering new colours and patterns I thought only to be imaginable.

I could have continued for hours but work had to be done. Monica kindly showed me the agates and Sicilian jaspers from the Earth Collection that I could choose to borrow. Having made my selection with Monica’s helpful guidance, and filled out the necessary paperwork, I returned back to the Courtauld Gallery with the specimens under my fierce protection. Glowing with joy, I now had everything I needed for the display.

The Oxford University Museum of Natural History

The Corsi Collection

Natasha Gertler and Monica Price with the Corsi collection

Stone slabs of Sicilian jaspers from the Corsi collection, corresponding to those present in the Courtauld frame

Illuminating Objects intern Natasha Gertler tells us about how her research is going.

As previously touched upon in my last blog post (and frankly, contrary to my natural instinct to apply analysis to any given problem) identifying decorative stones, such as those in the Courtauld’s ornate frame, is actually more an art than a science.

Whereas geologists can use destructive techniques to analyse and accurately identify rocks and minerals from the field – smashing and slicing them to view under a microscope – this of course is not an option when considering valuable objects from a museum collection. Even most non-invasive analytical techniques are inappropriate as the majority of these aim to establish information about the chemical composition and arrangement of a sample but many materials, both natural and manmade, have essentially the same chemical make-up. For example, both quartz and glass are composed of silica (SiO₂) and so cannot be distinguished by such analysis.

Therefore, identifying decorative stones is normally achieved through a mixture of observation, connoisseurship and comparison to stones of known identity. Awareness of such stones, their appearance, uses and provenance, is hence essential for identification. However, this is rather a niche discipline as it lies somewhere between geology and art history but is never fully taught in either field, but instead gained through experience and exposure. There are only a handful of such experts in the UK.

Dr. Ruth Siddall, Senior Lecturer in Earth Sciences at UCL, along with her student, Nadine Gabriel, kindly came to the Courtauld to lay eyes on the frame and carry out a bit of detective work. Armed with a microscope and a wealth of decorative stone knowledge, Ruth corrected our prior suspicions of the inclusion of man-made imitations in the frame, reassuring us that all the segments of the frame were indeed rocks and minerals sourced from around the 17th century.

Ruth Siddall and Nadine Gabriel observing the frame

Microscopic camera taking images of the frame

Natasha Gertler and Nadine Gabriel looking at the microscopic images of the frame

Ruth’s certainty arose by comparing the stones in the frame to the extensive Corsi collection. Housed in the Oxford University Museum of Natural History, it is composed of 1000 polished stone slabs of uniform shape and size (145 x 73 x 40 mm). By referring to the online catalogue, she identified the presence of various Sicilian jaspers in the frame.

In particular, Ruth identified what we initially thought was an imitation of tiger’s eye to in fact be a distinct variety of Sicilian jasper characterised by its yellow and black banded appearance. Shown below are images of the Sicilian jaspers in the frame (left) alongside the corresponding samples from the Corsi collection (right).

Ruth Siddall using the Corsi collection online catalogue for comparisonWith such an excellent resource on our doorstep, naturally our next step on the quest for identity was to contact Monica Price, Head of Earth Collections at the Oxford University Museum of Natural History and author of The Corsi Collection website.

Read all about my collaboration with Monica Price and trip to the Oxford University Museum of Natural History in my next blog post.

P.s. As the major contributor of information to the London Pavement Geology website, Ruth also pointed out to us the decorative stones used just on the doorstep of the Courtauld. These are Grey Oland Limestone and Red Oland Limestone arranged in a checkered pattern. Admittedly, I have never noticed these details, so next time you come and visit us make sure to spot the ancient fossils under your feet!

Our Illuminating Objects project for 2017 is now underway. In a series of blog posts postgraduate intern Natasha Gertler will share her discoveries as she chooses and researches the next Illuminating Object.

With an academic background in physics and an unwavering curiosity for crystals since, well, forever, it was only natural for me to choose the intricate frame bearing the double sided Aertsen painting for my ‘Illuminating Objects’ internship. Acquired by Count Seilern in 1969 from Alessandro Orso in Milan, the frame is beautifully inlaid in pietra dura with a multitude of semi-precious stones.

Or so we thought…

Upon initial inspection (which, by the way, was true love at first sight) and discussion with Sacha, Curator of Sculpture and Decorative Arts with whom I am working on this project, and Graeme, Chief Conservator, we had speculations of the material composition of the frame; that the central upper disc showing concentric red and white rings was pink agate, the two purple columns either side of the painting were amethyst and the several rich blue segments, lapis lazuli. But, as with most scientific investigations, things are not often as they seem and we needed to perform analytical experimentation to be sure.

In order to gain some insight into the constituent elements of the stones, we ventured down to the conservation laboratory of the Courtauld to use the X-ray Fluorescence (XRF) facilities, with the expert guidance of Professor Aviva Burnstock and Silvia Amato.

Having used spectroscopy many times in my undergraduate physics experiments (albeit on far less beautiful objects!) I felt quite at home and even a little nostalgic in the Courtauld’s lab. XRF works by shooting X-rays at a sample and then analysing the energies of the X-rays reflected back to the detector, which are characteristic to each individual element.

As expected, areas of red colour showed a dominance of iron (Fe) in their spectrum, and the dark blue areas exhibited the presence of Calcium (Ca) which is typical of lapis lazuli. However, many different stones as well as manmade materials also have several elements in common with each other. For example silicon (Si) is present in both agate and glass. Therefore, XRF cannot be used as a conclusive method of material identification, but rather as a further analytical tool.

Red, round element in the central upper area. Presence of silicon and iron.

For that reason, we turned to visual analysis instead. Dr. Emma Passmore, a senior teaching fellow in Earth Sciences at Imperial College London with experience in object analysis for the British Museum, kindly came to the Courtauld’s depot to help us identify these mystery materials. She agreed with our initial theories and added that the mirroring light blue elements on the lower panel were also agate.

However, she also suggested that some of the segments might not be semi-precious stones at all, but rather synthetic imitations. These included for example the panels that look like tigers eye and the red ovals at the very bottom, as well the orange and purple coloured banners across of the top and bottom of the painting, respectively.

Professional geologists identify stones by smashing and slicing them to make slides which they can then analyse under a microscope to a very high degree of accuracy. However, in the case of historically important and valuable objects such as this unique frame, this of course is not an option and alternative, non-intrusive methods must be used instead.

Will we ever be able to know the materials present in the frame with absolute certainty? Perhaps. And so the quest for identity continues…